-Batman. Yes, he doesn't have any superpowers in the first place, but that's the best thing going for him.
-Captain Marvel (the DC one, not the Marvel one)- A wizard did it. Enough said.
-And this one's a video game example, but I love too much to forget it: The Fury from Metal Gear Solid 3.
He was an experimental Russian cosmonaut during the 60's, before any cosmic ray shielding had been invented. The cosmic rays caused brain/eye damage which resulted in hallucinations of bright lights, like explosions. (This is actually realistic--John Glenn has told anecdotes about the same thing happening to him.) The thing is, he saw them as he was falling from orbit, superimposed onto his view of the Earth. This led him to believe that he was seeing a prophetic vision of himself covering the entire world in flames. Now he is a raving pyromaniac who walks around in a fireproof space suit, hovering around in a jetpack and firing his flamethrower at anything that moves. So he pretty much wins that game.
Can anyone else think of some good origins?